1. Field of the Invention
The invention relates to a method and a network analyzer for measuring the group delay time in a device under test.
2. Discussion of the Background
The linear transmission behaviour of an electronic component has to be determined for many fields of application. In this context, the phase distortion of the electronic component to be measured is also relevant in addition to the distortion of the amplitude. If the device under test—device-under-test (DUT)—is excited with a high-frequency excitation signal, a phase delay and accordingly a time delay, the so-called group delay time, can occur in the device under test. The group delay time τG, which is relevant in the following description, is obtained according to equation (1) from the rate of change of the phase Δφ dependent upon the frequency Δf.
                              τ          G                =                                            -              1                                      360              ⁢              °                                ·                                    Δ              ⁢                                                          ⁢              φ                                      Δ              ⁢                                                          ⁢              f                                                          (        1        )            The particular problem with a group delay time measurement is the impact on the group delay time of additional influencing factors within the device under test which cannot be eliminated directly and which impair the quality of the measured result. These include primarily the phase-distorting effects of input and output lines of the device under test or, in the case of a frequency-converting device under test, the influences of an unknown, possibly even a drifting oscillator frequency of a local oscillator in a mixer on the group delay time of the device to be measured.
A method for determining the group delay time in an electronic component which compensates such unknown influences on the group delay time of the device under test, is known from EP 1 515 147 A1. In this context, the device under test is excited with an amplitude-modulated excitation signal, which originates from the modulation of a carrier realized as a Dirac comb with a purely sinusoidal modulation signal. The response signal measured periodically in the device under test after the excitation of the device under test with such an excitation signal over respectively constant time intervals using a transient recorder is transformed in a spectrum analyzer by means of a Fast Fourier transform into the spectral range. The phases are determined from the two spectral lines of the phase-distorted response signal disposed in the lower and upper sideband of the amplitude-modulated response signal in each case at the individual carrier frequencies of the Dirac comb. The group delay time with a given carrier frequency is obtained from the difference between the measured phases of the spectral lines of the response signal disposed in the upper and lower sideband with the respective carrier frequency minus double the phase of the modulation signal—equivalent to the phase difference between the spectral lines of the associated excitation signal disposed in the upper and lower sideband of the same carrier frequency—and scaled with the known frequency difference between the upper and lower sideband of the amplitude-modulated excitation signal and respective response signal.
The determination of the group delay time of a device under test according to EP 1 515 147 A1 disadvantageously requires a plurality of laboratory devices—signal generator, transient recorder, spectrum analyzer, synchronisation device—and, for the compensation of the unknown phase of the modulation signal, provides an additional calibration measurement at a reference-carrier frequency. In particular, the generation of the Dirac comb is cost intensive.